The Effect of Dominant Junction on the Open Circuit Voltage of Amorphous Silicon Alloy Solar Cells.
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THE EFFECT OF DOMINANT JUNCTION ON THE OPEN CIRCUIT VOLTAGE OF AMORPHOUS SILICON ALLOY SOLAR CELLS. ADAM H. PAWLIKIEWICZ AND SUBHENDU GUHA Energy Conversion Devices, Inc., Troy MI 48084, U.S.A. Abstract We have theoretically studied the role of the doped contacts on the open circuit voltage (Voc) of amorphous silicon, pin solar cells by numerical solution of coupled Poisson and current continuity equations for the bulk and the doped regions. The built-in potential (Vbi) in the cell was found to be split asymmetrically between the p+/intrinsic and i/n+ regions. The first one forms a p+/n junction (dominant junction) since the undoped intrinsic is slightly n-type; the latter forms a n/n+ or low/high junction. We found that Voc is determined by the built-in potential at the dominant junction (and hence by the conductivity activation energy of the p+ layer) and is fairly insensitive to the activation energy in the n+ region. If one dopes the intrinsic with phosphorus, Voc is even less dependent on the n+/i junction potential but becomes very sensitive to the activation energy in the p+ layer. However, when the i-layer becomes p-type by boron doping, the dominant junction is created at the n+/i interface and Voc then becomes insensitive to the quality of the p+ layer and is determined by the n+ contact potential. The results are interpreted by considering the modification of the electric field within the bulk by the junction potentials and the recombination mechanisms at the two interfaces. Introduction. It has been shown that the substitution of an amorphous p+ contact by a highly conductive, micro-crystalline p+ material resulted in a significant improvement of aSi:H solar cell performance [1]. The gain was both due to the optical and electrical effects. The wider band gap, p+ electrode absorbed less light and the low activation energy of the micro-crystalline layer translated into higher built-in potential, which subsequently contributed to better carrier extraction and thus to higher FF and Voc. However, a number of experiments with micro-crystalline, highly conductive n+ material as a back electrode of a-Si solar cell did not result in a greater open circuit voltage, nor in better FF although the built-in potential had been substantially increased. To understand better the device physics governing solar cell operation, we have extensively simulated structures with amorphous and micro-crystalline contacts and varied their activation energies. To ensure that only the electrical properties are affected we simulated the front illuminated electrode as a micro-crystalline p+ layer with the activation energyAEf=0.1 eV and varied the back contact from micro-crystalline state withAEf=0.1 eV through 0.3 eV and then amorphous with AEf =0.3 eV to 0.6 eV. The simulation was done under the red illumination to ensure uniform generation rate and to minimize the influence of optical properties of the front contact.
Results. The results of the simulation are shown in Figs la and lb. One can easily see that for the cell without dopi
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